1. Field of the Invention
The present invention relates to method and apparatus for reducing pressure fluctuations in the supersonic wind tunnel that make it possible to reduce pressure fluctuations at the test section induced by the pressure fluctuations existing in the tunnel circuit, and to obtain wind tunnel test results in high accuracy.
2. Description of the Related Art
In conventional practice, the operation range of a compressor of the continuous supersonic wind tunnel circuits are used to be determined at the design stage. A compressor is commonly designed to operate at high efficiency in the range of comparatively high Mach numbers at the tunnel test section, but the operating efficiency thereof at low Mach numbers is not necessarily high. FIG. 3 is a diagram in which modified flow rate q is plotted on the horizontal axis, compression ratio xcfx80 is plotted on the vertical axis, and the operating characteristics of a compressor are shown as an example, with a modified rotational speed n and operating efficiency (adiabatic efficiency) xcex7ad as parameters. The operating efficiency of a conventional supersonic wind tunnel is high (70% or greater) at Mach number of 2.5 but extremely low (about 40%) when the Mach number is 1.5, as shown by the symbol xcex94. In FIG. 3, k is the specific heat ratio of gas (1.4 in the case of air), Z is the compressibility factor (1.0 in the same case), R is the gas constant (29.27 m/K), Ts is the static temperature (330K), and Q is the flow rate at compressor inlet (m3/s).
Stream in the tunnel circuit contains much both vortex energy and acoustic energy at low efficient compressor operation. The presence of such energy in the stream is observed together with the occurrence of pressure fluctuations as a pitot pressure fluctuation rate of 0.1% or greater in the stream at an operating Mach number of less than 2.2, particularly in a range of low Mach numbers such as 1.5-1.8. The energy in the stream can be measured as pressure fluctuations at the test section where the model is placed as the subject of the wind tunnel test. Since the pressure fluctuations existing at the test section disturb the laminar boundary layer on the model and cause turbulent transition, they may hinder improvement of measurement accuracy and affect the wind tunnel test results.
When the operating Mach number in a supersonic wind tunnel is in a range of low Mach numbers such as previously described, the occurrence of pressure fluctuations in the sections of the tunnel circuit extending from the compressor to the test section can be observed by sensors placed in the tunnel circuit. In particular, based on the measurement results of the pressure fluctuations in the settling chamber entrance, the nozzle entrance, and the center of the test section, the essential part of the pressure fluctuations in the center of the test section is believed to be directly induced by pressure fluctuations generated by a compressor operated at low efficiency. In the aforementioned operating Mach number range, most of the energy transferred from the compressor to the fluid is converted to thermal energy and vortex or acoustic energy (sound), and the energy from the compressor is not fully converted to the stream drive energy in the supersonic wind tunnel. Even if the stream from the compressor is cooled in a heat exchanger, the thermal energy alone is absorbed, while the vortex or acoustic energy remains in the stream.
In a supersonic wind tunnel designed as previously described, the test results obtained at a low Mach number do not necessarily conform with the results from the flight tests, so pressure fluctuations caused by the vortices or sound existing in the stream such as those that have an effect on the wind tunnel test results must be removed as much as possible from the upstream portion of the test section in order for the test results in the supersonic wind tunnel to be effective and reliable. In view of the above, a problem to be resolved herein is to provide a design in which pressure fluctuations in a supersonic stream can be reduced and the Mach number necessary for a test section can be obtained in the range of low Mach numbers (for which a considerable flow rate is required) by focusing on the operating efficiency of a compressor.
An object of the present invention is to provide a method and apparatus for reducing pressure fluctuations in the supersonic wind tunnel circuit in which the reliability of wind tunnel test results can be increased by minimizing the pressure fluctuations existing at the test section, and by reducing the pressure fluctuations that affect the measurement results.
Aimed at resolving the aforementioned problems, the present invention relates to a method for reducing pressure fluctuations in a supersonic wind tunnel circuit with the return leg, wherein the stream produced by the compressor is passed through a tunnel circuit resistor installed upstream of the test section, the compressor is operated at a higher operation efficiency than the operation efficiency determined by the operating state that corresponds to the flow condition for the Mach number required for the test section in the absence of the tunnel circuit resistor, and the Mach number at the test section is made to conform to the required Mach number by the additional pressure loss of the flow brought about by the presence of the tunnel circuit resistor.
The device for reducing pressure fluctuations in a supersonic wind tunnel circuit with the return leg in which stream produced by a compressor circulates in a tunnel circuit containing a test section comprises operation control means whereby the operation of the compressor can be controlled based on operating characteristics, with operating efficiency as a parameter; and an tunnel circuit resistor placed downstream of the compressor and upstream of the test section, wherein the operation control means performs control functions whereby the compressor is operated at a higher operation efficiency than the operation efficiency determined by the operating state that corresponds to the flow condition for the Mach number required for the test section in the absence of the tunnel circuit resistor, and the Mach number of the flow in the tunnel circuit is made to conform to the required Mach number by the additional pressure loss of the flow brought about by the presence of the tunnel circuit resistor.
In the supersonic wind tunnel, the inefficient component of the work performed by the compressor at a low operating Mach number is consumed as energy by the vortices or sound in the stream passing through the compressor. The compressor is operated at an operating point at which the operating efficiency thereof is markedly increased without changing the Mach number at the test section by having a design in which the stream produced by the compressor is passed through the tunnel circuit resistor, and the resistance factor thereof is set to an appropriate level. Since the compressor is highly efficient, the ratio of pressure fluctuations (sound and vortex energy) resulting from inefficient work to the total energy in the stream decreases, and the effect on the test section is reduced. In existing supersonic wind tunnels, the operating point in particular is selected such that the operating efficiency thereof is higher than the operating efficiency corresponding to the Mach number required for the test section (the required test section Mach number), the additional pressure loss necessary for the tunnel circuit is calculated based on the operating point and the desired compression ratio and flow rate (evaluated by the required test section Mach number), and the resistance factor of the tunnel circuit resistor for obtaining this pressure loss is determined. By the placement of such an tunnel circuit resistor downstream the compressor and upstream the test section, the Mach number of the stream at the test section can be made to conform to the required Mach number while the compressor can be operated at high operating efficiency, and the pressure fluctuations in the stream can be reduced.
According to the method and apparatus for reducing pressure fluctuations in the supersonic wind tunnel circuit of the present invention, conversion to vortex or acoustic energy due to the inefficient operating component of the compressor can be reduced and the pressure fluctuations existing in the stream in the tunnel circuit can be minimized at the same time as continuous operation can be maintained, by operating the compressor at an operating point at which the compressor exhibits higher operating efficiency than in the absence of an tunnel circuit resistor. The tunnel circuit resistor itself, consisting of a mesh plate, also has the effect of reducing pressure fluctuations and noise. As a result, pressure fluctuations affecting measurement results at the test section can be reduced, and the reliability of the wind tunnel test results increased. Since the range of high Mach numbers at which operation is possible cannot be reduced much by the characteristics of the supersonic wind tunnel or the characteristics of the tunnel circuit resistor even when the tunnel circuit resistor shaped as a mesh plate is installed in the tunnel circuit, improvements to wind tunnel tests at low Mach numbers can be devised without affecting the results of wind tunnel tests at high Mach numbers by only performing small changes, such as installing tunnel circuit resistors in existing supersonic wind tunnels.